This invention relates to resonant mechanical systems.
In one use of such systems, called laser printing, a focused laser spot is raster scanned across a light sensitive medium to generate an image. To accomplish high-speed scanning along one dimension of the medium, the laser beam is deflected across by a mirror that is supported symmetrically between two colinear torsion bars to effect resonant rotational motion (see, e.g., Montagu, U.S. Pat. No. 4,502,752, issued Mar. 5, 1985). The beam is slow scanned along the other dimension by moving the medium itself. Practical symmetrical scanners of this design have an upper frequency limit of about 2500 Hz and are relatively complex to make.
In such symmetrical scanners (which are characterized by having an odd number of rotating masses), the centrally supported mirror may be subject to cross-axis vibration. In the fundamental mode of cross-axis vibration, the mirror can move at right angles to the axis without tilting.
It is also known to support a resonantly rotating mirror asymmetrically, for example by mounting it at the free end of a torsion bar, the other end being fixed to the base of the scanner. The armature of a driving element is attached in the middle region of the torsion bar. The stationary part of the driving element is in turn mounted on the fixed base. The torsion bar need not have a uniform cross-section along its length. Such asymmetrical scanners have performance advantages over symmetrical designs, but the mirror is subject to tilt (called wobble), which reduces the accuracy of the scanning. Asymmetrical designs by definition have an even number of masses and thus cannot be constructed symmetrically.